Effects: Constructs an object of class basic_ifstream, initializing the base class with basic_istream(&sb) and initializing sb with basic_filebuf<charT, traits>()) ([istream.cons], [filebuf.cons]), then calls rdbuf()->open(s, mode | ios_base::in). If that function returns a null pointer, calls setstate(failbit).
Effects: the same as basic_ifstream(s.c_str(), mode).
Effects: Move constructs from the rvalue rhs. This is accomplished by move constructing the base class, and the contained basic_filebuf. Next basic_istream<charT,traits>::set_rdbuf(&sb) is called to install the contained basic_filebuf.